University of Illinois

No. 2/April 3, 1998

Weed/Crop Competition: Factors to Consider

Weed management strategies attempt to limit the deleterious effects of weeds growing with crop plants. These effects can be quite variable, but the most common is competition for available growth factors (light, water, etc.). Those quantities of growth factors used by weeds are thus unavailable to the crop. If weeds are able to utilize a sufficient amount of some growth factor to the detriment of the crop, the result can be, and often is, an adverse impact on crop yield.

The most common method currently employed to manage weeds is the use of herbicides. Producers can choose from many available options, each with its own distinct set of advantages and disadvantages. In addition, there are several different methods for applying herbicides. Regardless of the herbicide or method of application, the goal of using a herbicide is to prevent the weeds from contributing to a crop yield loss, by reducing the amount of competition exerted by the weeds.

Recently, the concept of weed/crop competition has become a marketing strategy for several herbicide manufacturers. The point of debate tends to focus on when competition (from weeds) should be removed so crop yields (of corn or soybean primarily) are not affected. Some maintain that soil-applied, residual herbicides should be used to eliminate any earlyseason weed competition, while others propose that the crop can compete with weeds for some time without affecting yields. Which view is correct? What type of research activities are needed to determine how and when weed competition reduces crop yield? How should results of studies like these be interpreted?

At the 1998 Illinois Agricultural Pesticides Conference, Loyd Wax with the USDA/ARS at the University of Illinois presented a paper dealing with many factors associated with weed/crop competition. The remainder of this article contains Dr. Wax's discussion.

Those involved in managing weeds have long recognized how weeds can harm crop growth and productivity by competing for light, moisture, nutrients, and space, as well as by hampering harvesting operations, reducing quality of the harvested crop, and producing weed propagules, thus leading to future problems.

Numerous studies over the years have compared weed species and density in various crops and assessed the importance of (1) duration of competition and (2) time of weed removal. From those studies, some general guidelines have evolved as to relative competitiveness of weeds with various crops, the weed-free time needed following crop emergence, and the appropriate time of weed removal with postemergence treatments to preclude loss of crop quantity and quality. However, as tillage, planting, and weed management practices have changed over the years, the former guidelines regarding crop/weed competition perhaps should be revisited, in some instances modified, as new findings are reported. This paper will review crop/weed competition to include earlier as well as more recent results and to raise some concerns and questions about interpreting the data in several aspects of crop/weed competition.

Crop and cultural practices. Crops obviously vary greatly in their ability to compete with weeds, ranging from crops that provide essentially no competition to ones that are very aggressive competitors. However, this discussion shall be limited to the major field crops of Illinois, corn and soybeans, for which there is the greatest concern with weed management and perhaps the most questions raised in recent years. Early studies, with a variety of weed species, tended to show nearly equal competitive ability of corn and soybeans, with some differences. Very tall-growing weeds, if left for the entire season, were sometimes less competitive in corn than in soybeans, mainly because the weeds could overtop soybeans and cause greater losses due to shading. For shorter-growing weeds that rarely grew taller than soybeans, yield loss was less in soybeans than in corn, again due to the excellent shading provided by a healthy stand of soybeans.

Crop varieties and hybrids can vary substantially in response to weed competition, with those that canopy earlier and provide more shading being the most competitive. For the most part, this aspect has not been exploited to any great degree. A number of studies have shown that increasing crop populations within the row, up to a point, can increase the competitive ability of the crop, with no deleterious effect on crop growth or yield.

Crop row spacing and time of planting can greatly influence the crop's competitive ability. Especially for soybeans, reduced row spacings have increased the ability of soybeans to provide additional competition with weeds, so that (under current production practices) soybeans may be more competitive than corn. Conversely, when planted in wide rows, soybeans and corn are probably more nearly equal in their competitiveness. Time of planting for both corn and soybeans presently occurs earlier in the season than it did than several decades ago, but this practice does not always enhance the crops' competitive ability. Very early planting, combined with reduced or no tillage, allows for greater weed competition, as well as for different species of weeds to be present, than has historically been common. Clearly, weeds that are established at the time of crop emergence begin to compete with the crop earlier than weeds that emerge only after the crop emerges.

With modern production practices and herbicides, do corn and soybeans differ in their ability to compete with weeds? Conclusive evidence is lacking, but many speculate that any difference is minimal in most instances. However, soybeans, especially when vigorous varieties are grown at high populations in narrow (8- to 15-inch) rows, usually have an edge over corn in competitive ability, assuming that complete weed control is achieved with herbicides prior to crop canopy closure and that neither crop will be cultivated.

Weed variables. Weeds have been able to reproduce, survive, and compete for centuries, at least partially due to their diversity. Species of weeds, and sometimes biotypes within species, can vary greatly in various growth habits and ultimately in their ability to compete with crops. Germination patterns differ markedly and sometimes erratically, causingdifferences in potential for competition from year to year. Emergence and growth also vary from slow and even, to rapid and almost unpredictable. Different species and biotypes appear to respond differentially to various environmental conditions: Some years are "nightshade" or "smartweed" or "nutsedge" years; whereas, in most areas of the state, every year is a "foxtail" or "velvetleaf" year. Most recent years could be described as "lambs-quarters" and "pigweed" years, and who can dispute the increased prevalence of "waterhemp" years recently across much of the state?

Obviously, as demonstrated in many competition studies, weeds produce markedly differing amounts of growth per individual plant and reach widely varying heights. These studies have allowed the development of relative competitive indices that can be somewhat helpful in determining the severity of problems presented by stands of various weed species. For example, it obviously requires more foxtail plants than cocklebur or giant ragweed plants to produce the same degree of competition with corn or soybeans.

The density or population of weeds required to cause a consistent yield reduction in crops has been difficult to establish. Many research studies have addressed this issue and have helped to establish some of the thresholds and guidelines currently available. In general, corn and soybeans can withstand low populations of weeds throughout the season without suffering yield or harvest losses; losses tend to increase linearly with increases in weed population, up to some population level above which further increases in yield reduction tend to subside.

Establishing consistent thresholds or numbers of weeds that cause a specific yield reduction has been difficult to derive across many locations, years, and weather patterns. A soon-to-be-published report summarizing competition studies conducted across several states and over many years concludes that improved techniques may be needed to establish and refine thresholds because variation across locations and years almost always occurs and can be considerable. This finding really should not be surprising and most likely can be attributed to differences in environmental conditions, with special emphasis on weather patterns. General threshold guidelines would be possible, as long as a range of likely responses is given, and could cover a majority of situations.

Competition research. Numerous studies over the years have attempted to define the critical duration of weed competition in corn and soybeans and determine the optimal time to implement weed management operations. One type of study is designed to determine the early season weed-free interval that is needed before the crop can shade out later-emerging weeds and subsequently progress independently for the remainder of the season, with no loss of crop quantity or quality and without increased harvesting losses. Studies like these are especially useful in determining how long a soil-applied herbicide needs to be effective after planting the crop. In general, for many weed species encountered in corn- and soybean-production systems of the northern United States, this interval ranges from 3 to 6 weeks, with 4 to 5 weeks being the most frequent requirement. It is important to note in these studies that some initiated the interval at planting, while others began at crop emergence (a potentially significant difference, depending on the season and weather). Most of these studies were conducted with healthy crop stands in 30- to 40 inch rows, with the objective to obtain 4 to 5 weeks without weed competition, after which the weeds were kept under control by crop shading and one or more layby cultivations. In sharp contrast, cultivators today are not used nearly as much as they once were, and weed management after crop emergence is administered in the form of postemergence herbicides if soil-applied treatments do not last long enough.

It is also important to note that these guidelines were developed with good crop stands and, for the most part, with the most common row-crop weeds, the majority of which tend to emerge fairly uniformly, not in multiple flushes well into the season. As mentioned here and again later, a review of the available data will indicate that, in most studies, variation from year to year has been considerable, probably due to differing environmental conditions, making it very difficult (if not impossible) to set a specific weed-free interval that will be acceptable with all species and across all locations and years.

Another factor to consider is that many of these studies were conducted either by seeding unimbibed weed seeds at various times after crop planting or by removing natural weed populations as needed for a specified period of time. These two methods effect different results, and how these results compare with a herbicide treatment that lasts the same amount of time is not specified. Does a lower dosage of a herbicide still cause some growth inhibition of later-emerging weeds? Such unanswered questions point toward the need for extreme caution in pronouncing exact time periods applicable over a wide variety of conditions.

Another type of study is designed to determine how long weeds can remain in the crop and eventually be removed with no resultant deleterious effects on quantity and quality of crop yield. In previous years, this was important so producers would know how early one needed to cultivate between the rows, as many older herbicides were applied only in a band over the row. With the growing prevalence of broadcast, selective postemergence herbicides, studies like these became relatively more important for providing guidelines in timing postemergence herbicide applications. Until fairly recently, these types of competition studies were often conducted by growing various populations of weeds from crop and weed emergence until the weeds were removed, either mechanically or by hand. The weeds were removed at some point after either crop planting or emergence, or until certain weed heights or stages. As a general guideline, many of these studies tended to show that a moderate population of weeds could remain growing with the crop for up 3 to 6 weeks after planting and, once removed, cause little or no crop yield loss. Studies like these also appear to have considerable variation in results, so it is again difficult to set specific intervals that will be valid over widely diverse conditions.

In assessing these studies, one needs to consider what species were involved and what weed population was present. In general, higher weed populations should be removed earlier, while less-intense populations can be left to compete longer. From an applied standpoint, a problem with many competition studies is that only one weed species is considered, whereas producers' fields often contain a number of species at varying populations. Personal bias would be to place the most emphasis on total weed biomass present at crop flowering and fruiting as the best indicator of loss likely to result from competition. However, this stage is generally well past that when control is possible or even feasible, and herbicides undoubtedly should be applied before this stage in most instances. To reiterate, these types of studies are influenced greatly by the environment, which makes establishing concrete intervals arduous.

The results of these studies should also be examined closely with respect to how the competition (weeds) was removed. In some cases, weeds were removed by hand, allowing any weeds that emerged afterwards to grow, while in other cases weeds were hand-weeded throughout the season, to simulate season-long control. Modern day studies tend to focus on controlling either a single species or a mixture growing at whatever population is present in the field, by applying selective postemergence herbicides at various weed sizes or growth stages. In interpreting the results of these studies, it is important to note whether or not the herbicide(s) used possessed any soil bioactivity that may have providedsome control of weeds emerging following application. Additionally, the population, the mixture of weeds, or both factors are important to note. And it is of special importance to note whether or not the weeds were actually controlled completely. This point is important because any yield reduction noted and attributed to preapplication-competition stress might actually have been partially due to postapplication stress from uncontrolled weeds or weeds emerging after application.

Invariably, these studies lead to a range of intervals for weed removal that work effectively under various conditions. Personal bias would tend to recommend removal of competition at the average or even slightly earlier time because potentially adverse conditions might cause delays in herbicide application, resulting in weeds that would be very difficult to control. This approach may become especially important when dealing with weed species for which later emergence might be a problem with herbicides that do not have soil-residual activity. Under this scenario, the conservative approach might involve adding to the mixture a herbicide that has soil-residual activity. As will be noted in the next section, environmental conditions can cause significant variation in the results of studies like these.

Environmental conditions. Factors that probably contribute more to the variation in results than any others are soil and air temperature, along with soil moisture content and rainfall before, during, and after initiation of competition studies. Even the best planned and conducted studies can vary considerably from location to location and year to year, often because environmental conditions vary. These conditions affect weed emergence and growth, herbicide effectiveness, the competitive interaction between crop and weed, and the ability of the crop to recover from early weed competition once the weeds have been removed. Primarily because environmental conditions vary, one should be very cautious in setting precise guidelines for crop/weed competition, including thresholds for density, duration of weed-free intervals, and times of competition removal. It would seem prudent to establish ranges of densities, times, etc. and/or to operate on the conservative side in these matters.

Other considerations. The total effect of weeds on crop plants is more correctly termed interference, which is the total of competition plus allelopathy. Allelopathy can and has been demonstrated; but, with most of our soils and cropping situations in the Corn Belt, it is thought to be relatively minor and is very difficult to demonstrate. Thus I have elected to discuss mainly competition, which I think is significantly more important, as it deals with plants competing for light, moisture, nutrients, and perhaps space. I suspect, however, that in dense infestations of weeds (such as grasses in corn)--besides competition- allelopathy could be a contributing factor to yield loss.

Competition for light may be one of the most important factors in reducing yields, especially with weeds that grow taller than the crop. Moisture stress, especially during and after removal of a very dense population of weeds, may be extremely important in crop recovery. One would not consider nutrient stress as greatly important in the rich, fertile soils across much of the Corn Belt; however, in soils with low fertility or coarse texture, it may be important. Some research has demonstrated that weeds can exhibit luxury consumption of some nutrients to the detriment of the crop.

Regarding time-of-removal studies with herbicides, several questions remain for which there does not seem to be much information. What are the relative effects of weeds on crop growth and yield when the weeds are removed at some stage by (a) hand- or mechanical weeding, (b) a fast-acting herbicide, (c) a slow-acting herbicide, (d) a herbicide causing little or no injury to the crop, (e) a herbicide causing substantial, temporary injury to the crop, or (f) a herbicide with residual activity, relative to one with no soil residual? It appears likely that when these factors are interacting with environmental conditions, variable results are almost certain to occur.

Those involved with developing weed management systems need to remember that the whole subject of crop/weed competition, while seemingly not simple, is even more complex in the marketplace. That is, weed management decisions are made not only based on a true crop/weed competition situation but also on other factors. Yield and quality loss are not the only factors being considered by those making decisions. Some producers are concerned about harvest difficulties, while others may be concerned about leaving a few weeds that will contribute to the soil weed seedbank. Aesthetic thresholds are real as growers worry about personal pride and the opinions of family and neighbors. Landlord thresholds may be the lowest of all, sometimes necessitating weed control at much higher levels than required based simply on yield losses. Product guarantees and respray programs also have contributed to extraordinarily high levels of weed management expectations on the part of the producers.

Summary. Numerous studies have investigated crop/weed competition from a variety of aspects. The results of these studies can be helpful in making decisions about weed management, as guidelines can be prepared indicating, in general, the relative competitive ability of various weeds at various densities in our major crops. These studies also provide guidelines for duration of weed-free conditions needed after crop emergence and for the time when weeds should be removed with postemergence herbicides. Data are somewhat limited, however, on mixtures of several weed species in competition studies. Other types of concerns, such as perception of the producer, neighbors, and landlord, may be as important as yield-loss indications from crop/weed competition studies in determining the types of weed management systems implemented.

Aaron Hager (, Loyd Wax (, andMarshal McGlamery (mmcglame@, Department of Crop Sciences,(217)333 4424